The present invention relates to a method and apparatus for slicing a work, such as semiconductor ingot, to produce a semiconductor wafer.
Conventionally, there have been known slicing apparatus in which a work is placed in a central hole of a flat, circular, ringed blade member having an internal cutting edge at an internal circumferential periphery thereof so as to slightly cross over the internal cutting edge in an axial direction of the blade member, and is moved against the internal cutting edge in a radial direction of the rotating blade member to produce a wafer.
In such a slicing apparatus, the blade member is likely to flex during cutting operation, which consequently causes a deterioration in the machining accuracy. To eliminate the deterioration, various slicing apparatus have been proposed.
For example, Japanese Unexamined Patent Publication No. 1-182011 discloses: considering the fact that the flexing direction and flexure amount of a blade member vary in accordance with the rotational speed of the blade member, the flexure amount of the blade member is detected, and the rotational speed of the blade member is feedback controlled to reduce the flexure amount, thereby suppressing flexure of the blade member.
Japanese Unexamined Patent Publication No. 4-211909 discloses that an air ejector for ejecting air under pressure onto a surface of a blade member is provided to suppress flexure of the blade member by the pressurized air.
Japanese Unexamined Patent Publication No. 1-275010 discloses that a flexure amount of a blade member is detected and a work is moved according to needs along a direction of an axis about which the blade member is rotated, i.e., in a feeding direction of the work, during cutting operation so as to reduce the flexure amount so that the thickness of a wafer cut out from the work is kept at a constant value.
In Japanese Unexamined Patent Publication No. 1-182011, the flexure of the blade member is suppressed by controlling the rotational speed of the blade member. However, in view of the fact that there is a delay in time until the flexure amount of the blade member actually changes after the rotational speed of the blade member is changed, it is difficult to set the feedback gain at a large value. Accordingly, this feedback control cannot provide the high responsiveness. This feedback control inevitably accompanies some deviation, i.e., a flexure of the blade member, and cannot consequently prevent the work from being cut at a position displaced from the determined position.
It could be considered that even if the blade member is flexed during the slicing operation, wafers having the same sectional form and a uniform thickness might be produced as far as the blade member is flexed at the same flexure amount when slicing off each wafer from the work, for example, slicing off a convex wafer W as shown in FIG. 9A.
However, when an internal cutting edge of the blade member is dressed with a dress device to enhance the cutting performance and the slicing operation is carried out again by the dressed blade member, the flexure amount of the blade member will reduce at the time immediately after the dressing because the cutting performance is enhanced, and the wafer surface cut by the enhanced blade becomes consequently straight. This straight surface is different from the other convex surface of the wafer which is produced by the not-dressed cutting edge as shown in FIG. 9B. The straight surface and the convex surface result in differences in the thickness of the wafer.
Further, there is a limit in the flexure control based on the rotational speed of the blade member because the controllable rotational speed is limited. The flexure of the blade member cannot be reduced when the rotational speed is beyond the controllable speed.
These drawbacks cannot be eliminated by the slicing apparatus disclosed in Japanese Unexamined Patent Publication No. 4-211909 for the similar reasons as mentioned above.
In the slicing apparatus disclosed in Japanese Unexamined Patent Publication No. 1-275010, the position at which a wafer is sliced off from the work can be controlled. However, since the slicing operation is executed in the state that the flexure of the blade member is not substantially suppressed, the following drawbacks occur.
As shown in FIG. 10A, an internal cutting edge 11 of the blade member 10 is designed in such a manner that both sides of the cutting edge 11 are brought into contact with the grinding points P over an entire circumference thereof in the non-flexed state which ensures most excellent cutting performance. However, when the blade member 10 is flexed in an axial direction of the blade member 10, i.e., rightward, as shown in FIG. 10B, the grinding point is displaced from the point P to the point P' due to the flexure of the blade member 10. Accordingly, the cutting Performance becomes poor, thus impairing the cutting surface of a sliced wafer.
Further, when the blade member 10 is flexed greater, a gap between the expanding side surface of the blade member 10 and a corner 30a of the work 30 becomes smaller. In the worst case, a part of the expanding side surface of the flexed blade member 10 comes into contact with the corner portion 30a, thereby damaging the blade member 10.